1. Field of the Invention
This invention pertains to a method for recovering petroleum from viscous, petroleum-containing formations including tar sand deposits, and more particularly to a method for recovering viscous petroleum from viscous petroleum containing formations by steam flooding. Still more particularly, this application pertains to a method for preventing loss of steam transmissibility during steam flooding operations.
2. Description of the Prior Art
There are many subterranean, petroleum-containing formations throughout the world from which petroleum cannot be recovered by conventional means because of the high viscosity of the petroleum contained therein. The best known and most extreme example of viscous petroleum-containing formations are the tar sands or bituminous sand deposits. The largest and most famous tar sand deposit is in the Athabasca area in the northeastern part of the Province of Alberta, Canada, which deposit contains in excess of 700 billion barrels of petroleum. Other extensive tar sand deposits exist in the western United States and in Venezuela, and lesser deposits are located in Europe and Asia.
Tar sands are defined as sand saturated with a highly viscous crude petroleum material not recoverable in its natural state through a well by ordinary production methods. The petroleum or hydrocarbon materials contained in tar sand deposits are highly bituminous in character, with viscosities ranging in the millions of centipoise at formation temperature and pressure. The tar sand deposits are about 35 percent by volume or 83 percent by weight sand, and the sand is generally a fine grain quartz material. The sand grains are coated with a layer of water, and the void space between the water coated sand grains is filled with bituminous petroleum. Some tar sand deposits have a small gas saturation, generally air or methane, although most tar sand depoits contain essentially no gas. The sum of bitumen and water concentrations consistently equals about 17 percent by weight, with the bitumen portion thereof varying from about two percent to about 16 percent. One of the striking differences between tar sand deposits and more conventional petroleum reservoirs is the absence of a consolidated matrix. While the sand grains are in grain-to-grain contact, they are not cemented together.
Recovery methods for tar sand deposits are classifiable as strip mining or in situ processes. Most of the recovery to date has been by means of strip mining, although strip mining is economically feasible at the present time only in shallow deposits, usually defined as those wherein the ratio of overburden thickness to tar sand deposit thickness is around one or less. In situ processes which have been proposed in the prior art include thermal methods such as fire flooding and steam injection, as well as steam-emulsification drive processes.
Several serious problems are frequently encountered when thermal recovery processes involving steam injection are applied to viscous petroleum containing formations. One of the most critical problems is associated with losing fluid transmissibility. When saturated or supersaturated steam is injected into a petroleum containing formation which does not contain appreciable gas saturation it is difficult to maintain adequate fluid flow between the injection well and the production well. Even though sufficient gas permeability exist initially to permit steam injection into the formation, the transmissibility declines sharply with time for several reasons. Condensation of steam after it has traveled some distance into the formation is one major cause for loss of transmissibility due to the fact that the viscosity of the steam condensate is substantially greater than the viscosity of steam vapor. Also, viscous petroleum heated by the injected steam will experience a substantial viscosity reduction, but as the heated petroleum moves away from the vicinity of the injection well it loses heat to the cooler formation, and consequently the viscosity increases rapidly. Eventually the petroleum viscosity will have increased to the point where it becomes immobile and blocks flow of any additional fluids through the formation. This is an especially troublesome characteristic of the tar sand deposits. Once a plug or blockage by cooled, viscous petroleum occurs, the heated fluids, e.g. steam, cannot be brought into contact with the cooled, immobile petroleum, so the block cannot be corrected by continued steam injection.
Another serious problem encountered in the course of steam injection into viscous petroleum formations is rapid pressure decline after termination of steam injection due to cooling and condensation of the steam in the formation. Since fluid injection pressure is the principal energy source causing petroleum movement in steam flooding processes, loss of pressure will cause rapid production rate decline even through the petroleum is still sufficiently high and its viscosity sufficiently low that production could be maintained. Maintenance of steam injection beyond an optimum point in order to avoid pressure loss is expensive and wasteful of fuel used to generate steam.
Still another problem encountered in relatively thick viscous petroleum-containing formations is vertical stratification of steam injected into the formation, which is generally caused by the density difference between steam vapor and petroleum.
In view of the foregoing, it can be seen that there is a substantial, unfulfilled need for a method for conducting in situ separation and recovery of bitumen by steam flooding so as to avoid plugging of the formation or serious loss of transmissibility and/or premature pressure decline.